1. Field of the Invention
The present invention relates to a magnetic film used as, for example, the core layer of a thin film magnetic head, and more specifically, to a magnetic film forming method capable of forming a magnetic film excellent in magnetic characteristics by improving the material of a target used in a film forming apparatus.
2. Description of the Related Art
A soft magnetic material used as the core layer of the writing head of a thin film magnetic head, that is, the core layer of a so-called inductive head and a soft magnetic material used as the magnetic film (magnetic core) of a flat type magnetic element such as an inductor and the like are required to exhibit a high magnetic permeability, a high saturation flux density and a high specific resistance and have a low coercive force in a high frequency region.
Japanese Unexamined Patent Publication No. 6-316748 proposes a Fe-M-O alloy as a soft magnetic material excellent in the high frequency characteristics, where an element M is a rare earth element and elements such as Ti, Zr, Hf, V, Nb, Ta, W in the Groups IVA, VA and VI in the periodic table.
Table 1 of the publication shows the magnetic characteristics of a plurality of Fe-M-O alloys which have a different composition ratio and in which the element M is formed of Hf and the like.
It is preferable that a high frequency magnetic material has a high saturation flux density Bs, a high specific resistance .SIGMA., a high magnetic permeability .mu. and a low coercive force Hc.
Among the magnetic materials shown in Table 1 of U.S. Pat. No. 5,573,863, one of the high frequency soft magnetic materials which is particularly excellently used for high frequency is a Fe.sub.54.9 Hf.sub.11.0 O.sub.34.1 film.
A noticeable point of this magnetic film is that the ratio of O in the oxide comprising Hf and O is larger than the stoichimetrical value of 1:2 in HfO.sub.2. A specific resistance can be increased by increasing the composition ratio of O.
Incidentally, the Fe-M-O alloy film is formed by sputtering and vapor deposition. Any of existing sputtering apparatuses such as an RF 2-pole sputtering apparatus, a DC sputtering apparatus, a magnetron sputtering apparatus, an RF 3-pole sputtering apparatus, an ion beam sputtering apparatus, a confronting target type sputtering apparatus and the like may be used as a sputtering apparatus.
As described above, a large amount of oxygen must be contained in the Fe-M-O alloy film to improve the specific resistance .rho. thereof.
Reactive sputtering can be exemplified as a method of adding oxygen (O) to the magnetic film.
In the reactive sputtering, a Fe--Hf alloy, for example, is used for a target and sputtering is performed in an (Ar+O.sub.2) mixed gas atmosphere in which an O.sub.2 gas is mixed with an inert gas such as Ar or the like.
With this operation, Hf is bonded to active O and the composition ratio of O contained in the magnetic film can be increased or decreased by adjusting the flow rate of the O.sub.2 gas.
However, there is a problem in the reactive sputtering that it is very difficult to properly control the flow rate of the O.sub.2 gas and the reproducibility (stability) of a formed film is bad.
Further, there is also a method of using, for example, a magnetron sputtering apparatus and sputtering a composite type target which uses a plurality of chips comprising HfO.sub.2 to a Fe target in an Ar atmosphere, in addition to the aforesaid reactive sputtering.
In this case, a method of adjusting the composition ratio of the Fe--Hf--O alloy film is to change the number of the HfO.sub.2 chips. That is, when it is desired to increase the composition ratio of O, it is sufficient only to increase the number of the HfO.sub.2 chips.
Although the composition ratio of O is increased by increasing the number of the HfO.sub.2 chips, the composition ratio of Hf is increased at the same time and the composition ratio of Fe is abruptly decreased. As a result, there arises a problem that the saturation flux density Bs which greatly depends on the composition ratio of Fe is reduced.
When the composition ratio of the Fe.sub.54.9 Hf11.0O.sub.34.1 film which is excellent in the soft magnetic characteristics is examined, it can be found that the composition ratio of O is about 3 times that of Hf.
However, even if the Fe--Hf--O film is formed using the above composite type target, the composition ratio of O is not made about three times as large as that of Hf.
This is because that since HfO.sub.2 is used as the chip material, the composition ratio of O is originally only twice that of Hf and accordingly the ratio of Hf to O of the Fe--Hf--O alloy film having been formed cannot be made to about 1:3.
As described above, the reactive sputtering is bad in the reproducibility (stability) of a formed film. Further, a magnetic film having a composition ratio at which all the magnetic characteristics can be made good cannot be formed by a target using an oxidizing agent comprising Fe and Hf.